(1) Field of the Invention
The present invention relates to heat resistant polyaniline or derivatives thereof and a method of manufacturing the same, and more particularly to solid electrolytic capacitors using such heat resistant polyaniline or derivatives thereof as electrolyte and a method of manufacturing the same.
(2) Description of the Related Art
A solid electrolytic capacitor usually comprises a porous member of a valve action metal, such as tantalum or aluminum, as a first electrode (anode), an oxide film formed as a dielectric film on a surface of the first electrode and a solid electrolyte formed as a part of a second electrode (cathode) on the dielectric. The solid electrolyte has a role of electrically connecting the entire surface of a dielectric film in the porous member and an electrode lead and, from this standpoint, it is preferable for the solid electrolyte to have high conductivity.
The solid electrolyte is also required to have a function of healing electrical shorts that are caused by defects in the dielectric film. This means that metals having no dielectric healing function cannot be used as solid electrolyte irrespective of their high conductivity. To this end, a metal such as manganese dioxide is used which undergoes transition to insulator by such causes as heat generation due to short-circuiting currents.
Also, since the solid electrolyte is exposed to heat of 240 to 260.degree. C. when used as a capacitor mounted on a printed circuit board, it has been usual to use a substance (for example, manganese dioxide or the like) which has a heat resistance of at least 260.degree. C.
Specifically, the substance serving as the solid electrolyte of a solid electrolytic capacitor should meet the following three requirements: Namely, that
(a) it has high conductivity; PA1 (b) it has a dielectric healing function; and PA1 (c) it has a heat resistance of at least 260.degree. C.
Manganese dioxide which has been used as solid electrolyte, although having sufficient properties insofar as the dielectric healing function and heat resistance are concerned, has not always been sufficient as solid electrolyte of solid electrolytic capacitors concerning the conductivity (about 0.1 S/cm).
Recently, vigorous development of solid electrolytic capacitors is in progress, in which such conductive polymers as polypyrrole, polythiophene and polyaniline having a conductivity as high as 10 to 100 S/cm and capable of being readily formed at room temperature are used as solid electrolyte.
The conductive polymer usually has high conductivity and dielectric healing function, but is rather inferior in the heat resistance. Therefore, it has been necessary, with polypyrrole, for instance, to improve its heat resistance by using alkylbenzene sulfonic acid with alkyl group carbon numbers of 2 to 16 (Japanese Patent Application Kokai Publication No. Hei 2-119213) or aromatic sulfonic acid (Japanese Patent Application Kokai Publication No. Hei 2-58817) as its dopant.
However, even the use of such dopant does not permit sufficient heat resistance to be obtained.
In the case of using polyaniline, the obtainable capacitors as disclosed in Japanese Patent Application Kokai Publication No. Sho 62-29124 do not have sufficient capacitor characteristics because of low dopant concentration of solid electrolyte polyaniline and also as low conductivity as 0.1 S/cm and below.
There is a method of forming an electrolyte by using a soluble polyaniline solution as disclosed in Japanese Patent Application Kokai Publication No. Hei 3-35516. In this case, it is impossible to sufficiently cover a surface of enlarged dielectric film because of a high viscosity of soluble polyaniline solution, so that a capacitance of capacitor as designed cannot be obtained.
There is further a method of chemical polymerization using a selected dopant (Japanese Patent Application Kokai Publication No. Hei 6-29159). In this case, excellent high frequency characteristics and also excellent thermal stability at 125.degree. C. are obtainable. On the demerit side, however, the characteristics are extremely reduced even in a short period of time at temperatures of 230 to 260.degree. C. and above, and the soldering property obtainable is inferior.